Over the past two decades, global aquaculture production has seen significant growth, particularly in Vietnam with the use of air-breathing fishes. However, the sudden economic importance of air-breathing species, such as Pangasianodon hypophthalmus, has resulted in a significant knowledge gap surrounding their basic biology. A fundamental understanding of how these animals work and how they interact with their environment is vital for the establishment of data-driven policies and best practice and essential for optimizing their growth and production. The aim of this work is to provide the aquaculture industry with the knowledge base necessary to fulfill it’s potential in improving food security and meeting the sustainable development goals of our planet in the 21st century and beyond.

The content of this dissertation consists of several studies characterizing different aspects of adapting, both behaviorally and physiologically, to aquatic hypercapnia and are as follows: Chapter 1 explores the effect on pulmonary respiration when brachial CO2 exchange is disrupted either though aquatic hypercapnia or forced emersion in Polypterus lapradei. Chapter 2 compares how air-breathing is used differently in three Vietnamese aquaculture species when encountered with aquatic hypoxia and hypercapnia. Chapter 3 investigates the role of ecophysiology on the use of air-breathing in the avoidance behaviors to either hypoxia and hypercapnia in Po. lapradei and Pa. hypophthalmus. Chapter 4 uses blood stress bioindicators to assess if hypercapnia levels in Pa. hypophthalmus aquaculture ponds represent a significant physiological challenge. We found Pa. hypophthalmus does not respond from a respiratory, behavioral, or blood biomarker perspective when exposed to levels of hypercapnia typically found in aquaculture (34 mmHg CO2). This demonstrates the extreme tolerance of Pa. hypophthalmus to aquatic hypercapnia acquired though millions of years of evolution to its natural environment, as well as suggest a lack of potential benefit for regulating CO2 in aquaculture ponds.

Much of this research focuses on P. hypophthalmus due to its central role in aquaculture, yet several other air-breathing fish species were used as a comparative approach to investigate how uniquely evolved respiratory systems respond differently. My hope is that that is work can provide scientific data informing the impact of hypercapnia in aquaculture practices, and at the same time, demonstrate the diversity of evolutionary solutions to an environmental challenge that exist among a fascinating group of organisms, the air-breathing fishes.